How to install flue expansion joint and when to replace it? This has a rigid standard. You need to see it

How to install flue expansion joint and when to replace it? This has a rigid standard. You need to see it

Two days ago, I met the engineer of Party A of a power station project, and complained to me that the expansion joint on their desulfurization flue began to leak after less than 3 years. When I removed it, I saw that the trough of the bellows was full of cracks. After asking, I found out that in order to save trouble at the beginning of the installation, the general-purpose corrugated expansion joint was directly attached to the high-temperature flue, the tie rod nut was not loosened, and the direction of the guide tube was reversed. Do you think this will hold up?

The installation and replacement of flue expansion joint really can't rely on patting your head. From power station boiler to cement kiln tail, from desulfurization system to blast furnace in iron and steel plant, thermal displacement compensation is a shutdown or even a safety accident. Today, let's break and break, what hard levers are hidden in the standards for installation and replacement of flue expansion joints.

1. Selection and matching: don't use the general-purpose type to attack the high-temperature flue

Many on-site masters think that the expansion joint is just a telescopic tube? Wrong. Flue operating conditions vary widely, and the medium temperature, pressure and displacement direction determine which type to use.

• High temperature flue (> 400°C):Must be onHigh temperature axial expansion jointOrMetal rectangular expansion joint。 The internal corrugated pipe material of ordinary corrugated expansion joint is usually 304 or 316L, and the temperature resistance limit is about 300℃. Beyond this temperature, the creep strength of the material will drop by a cliff, and the corrugated pipe will directly bulge and fail.
• Rectangular flue (e.g. power plant air flue):useRectangular non-metallic expansion joint(Fabric fiber material) orMetal rectangular expansion joint. The advantages of non-metal are to absorb multi-directional displacement, reduce vibration and noise, but it ages quickly, so it has to be considered to replace it in about 3 years. Metals have a long life but can only absorb axial displacement.
• Desulfurization flue gas system:Corrosive medium (SO₂, H₂SO₄) plus low temperature humidity, ordinaryNon-metallic expansion joints (fabric fiber expansion joints)It won't last. This is the time to matchPTFE-lined hoseOrRubber PTFE compensatorOr add a layer of PTFE lining to the inner wall of the metal bellows to isolate the corrosive medium. Otherwise, just like the power plant case mentioned at the beginning, the desulfurization bypass expansion joint will crack in 4 years, because the medium directly corroded the bellows.

Special hose for vacuum、Direct buried (fully buried) type expansion joint、External pressure single axial expansion jointThese special models have their own applicable scenarios, and choosing the wrong model is equal to laying mines. In the existing product information in the station,Corrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryIt is a subdivision for different working conditions, so don't mix it.

2. Several key cards in the installation process

If you choose the right model, if you step on the pit during installation, it will still be useless. The following details, I have seen a long list of error cases.

1. Tie rod nut: preload or loosen?

When the expansion joint leaves the factory, it will generally have a positioning nut on the tie rod to limit the excessive displacement of the bellows during transportation and hoisting. Once installed in place,The set nut must be loosened, allowing the tie rod to act as a guide and restraint (seeFunction of expansion joint tie rod）。 But if you buyStraight pipe pressure balanced expansion jointOrCompound hinge transverse expansion jointThe adjustment method of the tie rod is different-some need to be preloaded first to compensate for the cold tightness, while others require complete release. How to judge? Look at the displacement indicating arrow on the product nameplate, or ask the manufacturer directly for technical parameters.

There was a cement factory project before, and the workers screwed the tie rod nut to death. As a result, as soon as the pipeline heated up, the tie rod deformed, and the bellows directly exploded. Alas, this is typical empiricism killing people.

2. Direction of guide tube: don't reverse

Internal to the expansion jointguide tube(also called lining cylinder), whose function is to guide the flow of media and prevent the bellows from being directly washed by high-speed airflow. The opening direction of the guide tube must be consistent with the flow direction of the medium during installation. How to tell? Most expansion joints are marked with an arrow indicating the direction of the medium flow (seeThe direction of the arrow of the expansion joint refers to）。 If it is installed backwards, the high-speed flue gas will directly flush the bellows, and it will wear out in a few months.

3. One-sided fixed welding is strictly prohibited

This is a point many installation teams will commit. They welded one end of the expansion joint to death first, then adjusted the other end to match, and as a result, the expansion joint was forcibly twisted and welded. The "implied requirements" not specified in the standard are:Both ends must be free to face each other at the same time, and forcibly pulling after single-side fixation and welding is not allowed。 ForCompound straight pipe bypass pressure balanced expansion joint、Curved tube pressure balance expansion jointFor this type of model with complex structure, the original position should be marked before disassembly or installation. If the misalignment exceeds 2mm, the bellows may become unstable.

3. Hard indicators of replacement cycle: don't wait for leakage before replacing

Expansion joints, like people, have a life curve. Different materials vary greatly in life:

• rubber compensator、Non-metallic expansion joint (fabric fiber): It ages quickly, and it usually starts to delaminate and embrittle in 2-3 years in outdoor environment. It is recommended to visually inspect the fibrous layers once a year and replace them immediately if they find delamination, bulging or local breakage.
• Metal bellows(e.g.Corrugated expansion joint for power station industry): The design life is usually calculated according to the number of fatigue times, and it is generally required to reach more than 1000 full-stroke cycles. However, if the medium is corrosive (desulfurization flue gas, chemical tail gas), the actual life may be greatly reduced. In the above-mentioned power plant case, the bypass expansion joint of the desulfurization flue gas baffle door cracked after 4 years of use, because the corrosive medium was notPTFE-lined hoseQuarantine. Later, they replaced it with PTFE-lined bellows and addedRubber PTFE compensatorAs an alternative, there has been no problem in 6 years.
• Sleeve type pipe expansion joint: The sealing packing will age, and adding sealing grease during operation can prolong the life, but generally the sealing assembly needs to be replaced in about 5 years.

So, when do you have to change? In addition to visually visible cracks, delamination, leakage, you can also refer toService life of expansion jointData in Q&A: The design pressure level, material thickness and medium temperature of metal expansion joints will affect the actual life. A simple judgment method: each time the machine is shut down for maintenance, use a caliper to measure the wall thickness of the bellows trough. If the thinning exceeds 20% of the original wall thickness, it has to be replaced even if there is no leakage.

IV. Risk list of replacement operation

After talking about when to change, talk about how to change it. Before cutting on fire, do at least three things:

1. Cut off media and purge clean。 If it is a flue gas system, it must be confirmed that the flap door is closed tightly (Flue gas baffle doorOrDouble-sealed single-axis circular baffle doorIt is best to bring a sealing wind to ensure zero leakage).Simultaneously check whether the smoke baffle door and the plug-in insulation door are stuck or not-Don't let the expansion joint take the blame for the valve. The root cause of many leakage accidents is that the valve is not closed tightly, the medium flows back into the maintenance area, and flash explosion occurs during cutting.
2. Stress release。 ForCompound hinge transverse expansion joint、Straight pipe pressure balanced expansion joint、Double hinge expansion joint of air-cooled island vacuum pipeBefore disassembly, mark the original displacement indicator ring and record the zero position. Otherwise, the reference cannot be found during reset, and the new expansion joint will be offset as soon as it is installed.
3. Unilateral fixed welding is strictly prohibited(emphasize it again).Large diameter thick wall expansion jointIt has a great weight, so temporary supports should be set up when cutting old parts to prevent the pipeline from suddenly collapsing after cutting.

V. Acceptance and inspection indicators

After the new expansion joint is installed, you have to do a pressure test. refer toMetal hose pressure standardGenerally, the hydraulic pressure test is carried out according to 1.5 times the design pressure, and the pressure is kept for 10 minutes, and the pressure drop does not exceed 5%. ForNon-metallic expansion joint(Fabric fiber), can't do hydraulic test (fear of deformation of fiber layer after absorbing water), usually do weld airtightness test or kerosene penetration test.

Later inspection, focus on two points:

• Metal Bellows: See if there are cracks in the trough。 Many cracks start from the inner surface of the trough, which can't be seen from the outside, and need to be illuminated with a flashlight or detected with an ultrasonic thickness gauge. Once you find it, change it immediately.
• Non-metallic compensator: see if the fiber layer is delaminated and bulged。 Press it with your hand, and it feels soft or peeling, which means it is aging.

Expansion joint failure, not necessarily its own problem. Many times it isFlue gas baffle doorOrManual plug-in insulation doorGetting stuck, causing extra torsional loads on the pipeline and screwing the bellows out. Therefore, don't forget to check the valve status synchronously during the inspection.Electric plug-in insulation door、Round Flap Door (Double Seal)These devices, whether the action is flexible or not, and whether there is dust accumulation on the sealing surface will affect the working condition of the expansion joint.

The installation and replacement of flue expansion joints, the standard provisions are all bloody lessons behind them. Don't slap your head by experience, go according to the standard, change when you need to change, and check when you need to check. It is not worth saving a moment of money, hitching the price of downtime or even hurting people.